The present invention generally concerns pressure sensors, and more particularly, concerns micro-machined absolute pressure sensors and a process for making the same.
The recent progress in micro-fabrication and micro-machining technologies is transforming the field of solid-state transducers, making possible the production of micro-electromechanical systems (MEMS). In general, MEMS refers to the integration of sensors, actuators, and electronics using techniques originating in the semiconductor industry to realize miniature, high-performance. low-cost, electromechanical systems, with minimum feature sizes measured in microns. Miniaturization of mechanical systems in this manner is particularly attractive, since micro-mechanical devices and systems are inherently smaller, lighter, faster, and usually more precise than their macroscopic counterparts. MEMS devices are typically developed using computer-aided design (CAD) techniques created to facilitate VLSI (very large scale integration) production, and are typically batch produced using VLSI-based fabrication tools. Like integrated circuits, MEMS devices are rapidly progressing toward smaller size, higher speed, and greater functionality. Furthermore, because of batch processing, another major benefit of MEMS technology is its ability to drive down component cost.
Examples of MEMS devices include miniature fluid-pressure sensors and flow sensors, accelerometers, gyroscopes, and micro-optical devices. With a projected market of several billion dollars, pressure sensors are among the most important MEMS devices. While a majority of the semiconductor-based pressure transducers employ piezoresistive elements, devices that measure pressure based on changes in capacitance have become the focus for new developments to achieve higher pressure sensitivity, lower temperature sensitivity, and reduced power consumption. As with typical capacitors, these devices generally include a pair of conductive elements that are separated by a space. One or both of the elements flexes in response to pressure variations, thereby causing the capacitance measured between the conductive elements to change.
A capacitive pressure sensor disclosed in U.S. Pat. No. 4,853,699 has a generally hat-shaped semiconductor membrane that is attached around its periphery to a substrate to form a sealed-reference cavity. A conductive pad is disposed within the reference cavity. When a voltage differential is applied between the conductive pad and the semiconductor membrane, a capacitive charge is stored by the device. As the external pressure changes, the semiconductor membrane flexes, reducing the distance between the conductive elements, and thus changing the capacitance of the sensor.
In the method disclosed in the above-noted patent, the semiconductor membrane is fabricated by first forming a post of an etchable material on the surface of the substrate. Etchable silicon dioxide ridges at a height lower than the post are then formed on the substrate extending inwardly to contact the post. Polycrystalline silicon is deposited over the post and ridges, and the substrate is then etched to remove the post and the silicon dioxide ridges, leaving the polysilicon. This process forms a cavity with a plurality of channels extending therethrough. In order to seal the cavity, the substrate is exposed to a gas or vapor atmosphere, which causes growth of a material in the channels, closing them.
In many applications, it is necessary to measure pressure with a very high resolution, over a wide temperature range (e.g., xe2x88x9225xc2x0 C. through 85xc2x0 C.). Additionally, it is often necessary to sense absolute pressure. In order to monitor absolute pressure, a pressure transducer must include a vacuum-sealed reference cavity. Such a reference cavity cannot be obtained using the method and structure disclosed in the above-identified patent. No prior art MEMS devices are known that can provide a high resolution, absolute pressure measurement. Accordingly, it would be desirable to provide a capacitive micro-machined absolute pressure sensor having a vacuum-sealed reference cavity. Additionally, it would be desirable to produce such a sensor using a batch process that reduces the number of processing steps and masks required, compared with prior art processing methods for producing similar devices.
In accord with the present invention, an absolute pressure sensor including a vacuum-sealed reference cavity and method for making the sensor by micromachining a silicon substrate is provided. The sensor includes a flexible semiconductor membrane defining a cavity that is bonded to a substrate. preferably glass or silicon, under a high vacuum to form a vacuum-sealed reference cavity using a combination of eutectic and anodic bonding techniques. A first conductor is disposed within the cavity, and a transfer lead extends therefrom through the cavity wall. A nonctonductive pad, preferably made of a dielectric material, is disposed between the transfer lead and the semiconductor membrane so as to electrically isolate the semiconductor membrane from the transfer lead. Additionally, a second conductor is connected to the semiconductor membrane. When a voltage differential is applied across the two conductors, a capacitive charge is built up between the first conductor and the semiconductor membrane. As the external pressure changes, the semiconductor membrane flexes, changing the distance between portions of the membrane and the first conductor, causing the capacitance to change.
The absolute pressure sensor is preferably fabricated using a batch manufacturing process. First, a plurality of cavities are formed in the top surface of a silicon wafer, preferably by applying a first mask to the silicon wafer and bulk machining the silicon wafer with a chemical etchant. A semiconductor layer is then formed over the top surface of the silicon wafer and the plurality of cavities through boron diffusion. A plurality of dielectric pads are then formed adjacent to corresponding cavities using a second mask. Next. a nonconductive bonding layer is formed over the semiconductor layer and the plurality of dielectric pads by deposition of polysilicon or amorphous silicon. Material in proximity to each cavity is etched away from the silicon wafer using a third mask so as to define a plurality of individual sensor membranes. Each membrane comprises a welled portion connected to a planar periphery upon which a dielectric pad is disposed. The silicon wafer is next turned over, and the peripheries of the membranes are bonded to a nonconductive substrate, preferably glass, on the upper surface of which are disposed a plurality of conductors arranged so as to provide a pair of conductors for each sensor membrane. The silicon wafer and the nonconductive substrate are bonded in a vacuum so that when the bonding layer of each membrane is bonded to the top surface of the nonconductive substrate, a plurality of vacuum-sealed reference cavities are formed, each reference cavity having a pair of conductors extending therefrom. One of the conductors extends into the reference cavity and is electrically isolated from the membrane by the dielectric pad, and the other conductor is in electrical contact with the semiconductor membrane. Preferably, the conductors are produced by depositing gold on the nonconductive substrate. If the nonconductive substrate is glass, an electrostatic bonding process is performed that causes the gold to migrate into the bonding layer and form a eutectic seal with the glass. The dielectric pad prevents the gold from the lead connecting the conductor disposed in the cavity from reaching the semiconductor membrane, while a portion of the gold in the other conductor, which comprises a contact, migrates through the bonding layer to form an electrical contact with the semiconductor membrane. After bonding is completed, excess portions of the silicon wafer and the nonconductive substrate are removed to define the final shape of the sensor.